Some GARTs may have addressing limits which can't cover all of the physical addresses which may be available to the CPU. This is an attempt to categorize chipsets.

The maximum memory is how much you can slot in. The memory controller address limit is how much address space the northbridge is decoding. So, 4 GB of address space with 4 GB slotted in will result in less than 4 GB of RAM available to the OS because a chunk of address space gets reserved for the PCI and miscellaneous apertures. The GART address limit is how much of the MC address space the GART can map to. Status is whether we need to worry about GART addressing as a result.